The first indicating light displays of the above type had the disadvantage that they did not provide a homogeneous light, despite various spreading arrangements for spreading the light which were used and which consisted generally of billets or toroids incorporated in the cover lens of the display, or in an optical plate of the display. Although this disadvantage is in general tolerable from the point of view of the statutory regulations, it is ugly from the aesthetic point of view, especially since, nowadays, increasing importance is being attached to the homogeneity of the light emitted by indicating lights of vehicles.
French patent specification No. 2 614 969 discloses an indicating light display with a single light source, giving homogeneous illumination. That particular display comprises, within a housing, a transparent or translucent optical screen which is referred to as a "supplementary lens", and which is interposed between a single light source and an optical plate. This plate is essentially flat and extends in the direction at right angles to the optical axis of the light display. The particular geometrical configuration of the supplementary lens ensures that the surface density of light flux arriving on the optical plate is essentially constant over the whole surface of this plate. Finally, the surface of the plate is formed with striations which redistribute, or redirect, the light rays emitted from the supplementary lens, so as to transmit to the outside of the vehicle a beam within the solid angle provided for in the regulations.
However, vehicle manufacturers have in practice designed stop lights with dimensions which are sometimes very large, and which make use of a plurality of light sources. It is clearly possible to conceive the use of a device that consists of optical sub-assemblies of the kind described in the above mentioned French patent specification, juxtaposed together and giving homogeneous illumination within each sub-assembly. However, in such an arrangement there would then be irregularities in the illumination at the junctions, or transition zones, between adjacent sub-assemblies.